1. Field of the Invention
The invention comprises a method and apparatus for generating soft X-ray laser action through the use of thin blades which help to remove heat from a magnetically confined plasma.
2. Description of the Prior Art
The possibility of producing a soft X-ray laser has attracted recent attention because of the interesting scientific potential for such a device. It is believed that an X-ray laser would have the capability of forming holographic interference images which could, for example, be used to study the detailed three dimensional structure of small molecules such as DNA. Other significant applications are anticipated. The potential for X-ray lasers is described in the Oct. 30, 1982 editions of THE NEWARK STAR LEDGER and THE WALL STREET JOURNAL as well as the Apr. 2, 1985 edition of The NEW YORK TIMES.
Experimental efforts have focused on a population inversion in CVI in a recombining plasma. The principle of the recombination technique is, first, to create and heat a multi-Z high density plasma in order to ionize atoms to the proper stage of ionization and, second, to cool the plasma rapidly to create a strongly non-equilibrium (recombination) regime. In such a regime, the ions recombine with the electrons by a three-body process with a rate that is the highest for highly excited levels (high n-quantum number). The lower levels are populated by collisional cascading processes. Hence, in a recombining plasma the process of population occurs from higher to lower levels. At the same time, depopulation of low-n levels is predominantly by radiative (spontaneous) transitions; in hydrogen-like ions this has the highest rate for the resonance 2 to 1 transition. In this way, in hydrogen-like ions for example, a population inversion can occur for levels n=3 and 4 relative to level 2, with a higher gain on the 3 to 2 transition (in CVI this transition corresponds to a wavelength of 182.2 Angstroms).
Most of the research on the recombination scheme has been based on fast adiabatic cooling due to free expansion of the plasma into a vacuum or cooled gas. At the same time, there are other theoretical and experimental studies of other approaches to soft X-ray laser development with credible results, most recently from Livermore in electron - collisionally pumped neon-like Se. One advantage of the recombination scheme is a relatively rapid decrease of wavelength with (isoelectronically) increasing charge of atomic core Z of the working element. However, the disadvantages of the scheme based on plasma recombination during free expansion are the rapidly decreasing electron density, the difficulty of obtaining a relatively long and uniform plasma column and control of the cooling rate. To minimize these problems, it has been proposed to create a plasma column with a relatively large ratio of plasma length to radius (ratio greater than or equal to 100) in a strong solenoidal magnetic field and to cool it by radiation loses. See Suckewer and Fishman, "Conditions for Soft X-Ray Lasing Action in a Combined Plasma Column", J. Appl. Phys. Vol. 51, No. 4 April 1980, 1922- 1931. Because the spectral line radiation losses are proportional to Z.sup.4, and three body recombination is proportional to the square of electron density n.sub.e.sup.2, such cooling is believed able to produce very rapid recombination in a plasma with high enough Z and n.sub.e. It is also belived that radiation cooling of a magnetically confined plasma column can be more efficient than adiabatic cooling of an expanding plasma. See Suckewer et al, "Population Inversion and Gain Measurements for Soft X-Ray Laser Development in a Magnetically Confined Plasma Column" IEEE Journal of Quantum Electronics, Vol. QE-19, No. 12, December 1983, 1855-1860. The magnetic field would prevent the radius of the column from increasing and hence prevent a rapid decrease in electron density.
The patent literature also includes discussions of efforts to produce soft X-ray lasing from plasmas. In many respects U.S. Pat. No. 3,823,235 entitled X-RAY LASER is typical of the patent prior art. A short pulse laser having a pulse length in the neighborhood of 10.sup.-12 seconds is focused by an optical network onto the end of a filament having a diameter of up to about 1 micron and a Z in the range of between 2 and 30. In this environment a short time scale is necessary in order to avoid depopulation of excited levels by spontaneous emission. That invention differs from the present invention in that the present invention operates on a much slower time scale and the approach described in U.S. Pat. No. 3,823,325 is not based on plasma recombination as defined in this disclosure.
U.S. Pat. No. 3,961,197 entitled X-RAY GENERATOR is of interest in that it includes a discussion concerning the use of a magnetic field to confine a plasma in the context of X-ray generation by laser pumping. In one embodiment a primary X-ray burst causes a mixture of high Z and low Z gases to produce coherent X-rays. The primary X-rays ionize the lasing medium. Charge-exchange with neutral atoms then produce the population inversion necessary for producing coherent secondary X-rays. Two essential features of that invention are the conversion of primary X-rays to secondary X-rays and electron transfer from low Z atoms to high Z atoms. Neither of those features are considered to be parts of the present invention.
U.S. Pat. No. 4,229,708 entitled X-RAY LASER discloses the use of the CO.sub.2 laser which bombards a lithium target. The patent further discloses a variety of different elements with an atomic number Z ranging from 3 to 10 which might also be suitable. One of the materials identified as being a suitable equivalent is carbon V. There is also some discussion concerning the role of aluminum. That invention is based upon excitations of populations in metastable levels and the subsequent extraction of energy from these levels by spontaneous anti-Stokes scattering. However, the invention set forth in U.S. Pat. No. 4,229,708 is otherwise very different than the present invention. The present invention does not use the 1s2s metastable levels nor anti-Stokes Raman scattering which are an essential part of the disclosure of U.S. Pat. No. 4,229,708.
U.S. Pat. No. 3,746,860 entitled SOFT X-RAY GENERATOR ASSISTED BY LASER describes one of the earlier efforts to use a CO.sub.2 laser to generate soft X-rays. That disclosure appears to be directed primarily towards the use of high Z material. The laser is used to enhance spontaneous X-ray emission. Therefore, U.S. Pat. No. 3,746,860 does not describe a true X-ray laser. The present invention, however, causes the generation of stimulated X-ray emission and is therefore absolutely different from the device described in U.S. Pat. No. 3,746,860.
U.S. Pat. No. 4,143,275 is of interest only in that it discusses the use of a wide range of high Z numbers. It primarily discloses a device for reflecting X-rays from a conventional X-ray source. The references to sources of stimulated emission involve the old idea of inner shell photo-ionization. The patent appears to be directly solely towards an apparatus for reflecting X-rays and not towards an X-ray laser as such.
Several prior art patents discuss the use of aluminum as a target or part of a target. For example, U.S. Pat. No. 3,826,996 entitled METHOD OF OBTAINING A MEDIUM HAVING A NEGATIVE ABSORPTION COEFICIENT IN THE X-RAY AND ULTRAVIOLET SPECTRAL RANGE AND A LASER FOR PRACTICAL APPLICATION OF SAID METHOD discloses the use of an aluminum target which is impinged upon by a laser beam emitted from a giant-pulse laser. That invention is based upon population inversion between metastable energy states and low energy states in a laser produced plasma. An essential characteristic is that the plasma is chosen so as to insure that there should exist metastable states which can be populated. That invention is different from the present invention in that metastable states play no role as such in the present invention.
Similarly, U.S. Pat. No. 4,504,964 entitled LASER BEAM PLASMA PINCH X-RAY SYSTEM discloses the impingement of X-rays from a plurality of laser sources onto an aluminum target.
Finally, the following patents may be of interest with regard to the generation of X-ray lasers in general, but otherwise do not appear to be relevant to the apparatus and method of the present invention: U.S. Pat. Nos. 3,484,721; 3,518,427; 3,617,929; 3,813,555; 3,864,643; 3,882,312; 3,967,213; 3,972,008; 4,012,640; 4,034,226; 4,042,827; 4,053,783; 4,058,486; 4,201,955; 4,206,364; 4,218,628; 4,317,994 and 4,380,072.
Insofar as understood none of the prior art teaches or suggests the method and apparatus of the present invention including the novel structure of the target means employed to obtain enhanced soft X-ray lasing action.